(1) Field of the Invention
The present invention relates to a process for preparing LiCoO.sub.2 or LiNiO.sub.2 used as cathode material in a lithium secondary cell, particularly to a process for preparing LiCoO.sub.2 or LiNiO.sub.2, which is microscopic and a high specific surface area cathode active material with a chelating agent, polyacrylic acid(PAA), malic acid or polyvinyl butyral (PVB) at low temperature and for a short time.
(2) Description of the Related Art
New portable electronic equipment such as an integrated camera-VTR, audio, lap top type personal computer and cellular phone have been widely used since their introduction into the market. Along with the market expansion, technology has rapidly increased as to make such products smaller and lighter. Therefore, to meet the power needs of the above mentioned products, the demand has been rising for the development of an intelligent battery and power supply with a high density/capacity performance. Generally, a battery is classified as a primary disposable battery such as; Mn battery, Alkaline battery, Hg battery and oxidized Ag battery or a secondary rechargeable lead storage batteries such as; low voltage Ni/MH battery (Metal hydrides used as cathode active material), enclosed Ni-Cd battery, Ni/H.sub.2 battery, Li grouped batteries (Li metal battery, LIB: Li ion battery, LPB: Li polymer battery) fuel battery, and solar battery.
Among these two classifications, the primary battery has some problems. Namely, the life span is short, and environment pollution occurs because the primary battery is not reusable. On the other hand, a secondary battery has several advantage over a primary battery; the first advantage is having a longer life span and producing less waste, thereby causing less environmental pollution, since a secondary battery can be recharged and reused, and the second advantage is having better performance and efficiency than a primary battery because the average voltage (3.6 V) of the secondary battery is significantly greater than the average voltage (1.5 V) of the primary battery.
Among the secondary batteries, the Ni/Cd is the most widely used. But, recently, research for Li grouped secondary batteries has been vigorously progressed, because of capacity limitation due to heavy weight and large volume, and because of the decreased memory effect and total capacity due to a non-complete discharge. Compared to the prior Ni/Cd battery, the weight and volume of the Li grouped batteries are less than half the Ni/Cd battery. Thereby allowing very high energy density. In addition, Li grouped batteries have another advantage of being semipermanency.
The Li grouped secondary batteries as mentioned above are composed of a cathode that has 3-4.5 V more than Li/Li.sup.+ electrode potential, an anode made of Li metal or material composed of Li metal (Li.sub.x C.sub.6, Li-Al, Li-Pb, . . . ), and electrolyte. Among the said cathode materials, the cathode materials of the Li secondary battery that is currently used or has a usable potential include transition metal oxides (LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2 O.sub.4) and oxide solid solutions (LiM.sub.x Co.sub.1-x O.sub.2, M.dbd.Ni, Co, Fe, Mn, Cr, . . . ) and the like. Recently, in fact, a Li ion secondary battery manufactured by Sony Energytec Company and Moli Energy Company is composed of a carbon material as an anode active component, LiCoO.sub.2 and LiNiO.sub.2, as an cathode active component for Sony Energytec Company and Moli Energy Company respectively, and PC/DEC/LiPF.sub.5 (PC: Propylene Carbonate, DEC: Diethylene Carbonate) as an electrolyte. The LiCoO.sub.2 amongst the said cathode material is widely used now. And, recently, it showed good performance when used as an alternative cathode for melting carbonate salt. Furthermore, another advantage, is that the cost of production of LiNiO.sub.2 is lower that any other cathode material.
However, because the capacity of the currently used cathode material of 140-270 mAh/g is relatively low compared to the theoretical total capacity value of 372 mAh/g for the Li.sub.1 C.sub.6 Standard, and only 20-30% of the theoretical total capacity is available with the current technology level. Therefore, efforts to improve the prior cathode material is attempting to avail further theoretical capacity in parallel to developments of new cathode material that has a higher capacity. In general, the quality powder of cathode material is determined by a good crystal property, a homogeneity, a homogeneous powder morphology having narrow particle distribution, and specific surface area. The reason is based on the fact that while charge-discharge occurs, the charge of microstructure is inhibited, and also, the more the cathode material has a specific surface area, the more it has reactive area.
The most general method for the preparation of LiCoO.sub.2 or LiNiO.sub.2 widely used as a cathode active material in a Li secondary battery like the said material is solid phase reaction method. This method performs several cycles in the process of mixing and calcining of the raw materials, namely carbonate salt and hydroxide of each component. But this method, in the case of preparing LiCoO.sub.2 or LiNiO.sub.2, allows a large quantity of contaminants in the mixing stage of the raw materials thereby making it difficult to obtain a homogeneous phase, since non homogeneous reactions occur more easily, and uniform control of the size of the powder. As a result, the sintering property is dropped, and the specific surface area of the powder is lowered due to a high temperature and a long process time is needed and the use of the resultant prepared LiCoO.sub.2 or LiNiO.sub.2 in this way as a cathode material causes the problems that the electrode performance is lowered.